This invention relates to a speed ratio control system of a continuously variable transmission for a vehicle, wherein a speed ratio is feedback-controlled in association with a deviation between a target and a measured value, so that the speed ratio can be continuously varied and adjusted, more particularly this invention relates to improvements in a speed ratio control system of a continuously variable transmission for a vehicle, wherein better shift characteristics can be obtained by improving a shift speed or a gain of feedback control.
In general, continuously variable transmission mechanisms include V-shaped pulley devices, each including a stationary pulley and a movable pulley, and each having an effective diameter which can be varied by hydraulic servo devices. These V-shaped pulleys are provided on input shafts and output shafts respectively, so that rotation of the input shaft can be shifted in the continuously variable manner and transmitted to the output shaft by a driving belt extending between the V-shaped pulley devices. Normally, an oil flowrate to the hydraulic servo device on the input side is varied by a flow control valve, whereby the effective diameter of the V-shaped pulley device on the input side is forcibly changed. On the other hand, hydraulic pressure of the hydraulic servo device on the output side is varied by a pressure control valve to thereby follow the change of the effective diameter of the V-shaped pulley device on the input side, so that the driving belt will not slip in transmitting the torque.
As compared with automatic transmission mechanisms, which consist of so-called torque converters with groups of planetary gear units, the above-described continuously variable transmission mechanisms are advantageous in that abrupt changes in driving force during running of the vehicle are reduced, shift shocks are low, and the fuel consumption rate is good. In recent years, there is an increased demand for further improvements in continuously variable transmission mechanisms.
The speed ratio control of continuously variable transmissions has heretofore been performed as described below. First, a target value (normally, a target rotational speed Nin.degree. or a target speed ratio e.degree.) is calculated, a deviation D=Nin.degree.-Nin (or D=e.degree.-e) is calculated, and, in association with this deviation D, an oil flowrate Q (=a control voltage Vin of the flow control valve, being commensurate to the oil flowrate Q) to the hydraulic servo device on the input side is calculated through an equation Vin =K.multidot.D. K is a feedback gain and has heretofore been set at a constant value. Furthermore, this control voltage Vin has determined the oil flowrate at the flow control valve, and in turn, determined the shift speed. As a consequence, heretofore, the feedback gain K has been set at a constant value, and, the shift speed has been also inevitably determined in association with a deviation between a target value and a measured value.
However, the conventional control described above presents a disadvantage in that it may not satisfactorily smoothly transfer the measured value to the target value during the transitional period and stably maintain the continuously variable transmission at the target value at steady state.